Devices for capturing radiographic images are generally comprise two main components. One is an X-ray tube adapted to generate a beam of X-rays along a defined axis, and the other is an X-ray receiver disposed in the path of the X-ray beams to capture the X-ray. The X-ray receiver can be provided with a film, a storage phosphor sheet/cassette (Computed Radiography or CR), or an electronic device (Direct Radiography or DR). The anatomy to be studied is arranged between the X-ray tube and the receiver. When X-rays pass through the anatomy and impinge the receiver, a radiograph image of the anatomy is generated.
In routine assessments of radiographs, the relative position of patient during exposure is typically desired. Consequently, a marker with letters or symbols made of lead is often used. The marker is arranged on the path of the X-rays, for example on the receiver, so that the radiograph bears the mark of these letters/symbols. Generally, markers are made by using standardized abbreviations, indicating whether the left or right part of the patient's body is being radiographed, which projection is being taken, and/or the patient's position. In some situations, the marker may also contain symbols representing the hospital, institute or clinic site, conducting the radiographic study.
It is desirable to identify the marker in a radiograph, since the marker provides useful information for correctly displaying the radiograph. For instance, the left and right markers are usually used to identify the side of the body part. If such information is ignored and an image is incorrectly displayed, a misdiagnosis can cause serious damage. In the case of chest radiography, wrong side information may lead to performing a biopsy not on the lung presenting a lung nodule or other symptoms, but on the other healthy lung.
Another advantage of recognizing/identifying the marker is that it helps to provide the information needed for storing and managing images in picture archiving and communication systems (PACS), radiology information systems (RIS) and hospital information systems (HIS). For example, in mammography, a marker is generally needed to specify the projection during image acquisition. In order to appropriately store and retrieve image, the projection is also required in DICOM header. Currently, this information is manually input by technologists. Applicants have recognized that if the marker can be automatically recognized and filled into the DICOM header, it would reduce the input time and incidence of mislabel or unlabeled images, therefore, greatly improve the workflow.
Egmont-Petersen et. al (“Recognition of radiopaque markers in X-ray images using neural network as nonlinear filter”, Pattern Recognition Letter, Vol. 20, pp 521-533, 1999) developed an approach for recognizing of markers in cineangiographic images based on neural networks. However, their markers are small gold spheres, and always appear as circular-symmetric shapes in images, which are totally different from the markers mentioned in the present invention. Moreover, their markers serve as landmarks for locating anatomical structures; in contrast, radiographic image markers can be used to identify the examination conditions and usually have the semantic meanings associated with them. Due to the different purposes, Egmont-Petersen's approach is not suitable the present problem setting.
U.S. Pat. No. 6,354,737 issued Mar. 12, 2002 to Hufe et al. entitled DIGITAL IMAGE ORIENTATION MARKER discloses a method for generating an orientation marker for a digital radiogram after exposure. This marker is formed by a plurality of pixels arrayed along a plurality of rows and columns of pre-defined patterns. Therefore, it is not necessary to recognize/identify the marker.
In the field of image recognition and retrieval, some approaches have been proposed to identify trademarks. For example, Jian et. al (“Shape-based retrieval: A case study with trademark image databases”, Pattern Recognition, Vol. 31, No. 9, pp 1369-1390, 1998) proposed a method for trademark image database retrieval based on object shape information that would supplement traditional text-based retrieval systems. Hsieh et. al. (“Multiple classifiers for color flag and trademark image retrieval”, IEEE transaction on image processing, Vol. 10, No. 6, pp 938-950, 2001) presented a region-based multiple classifier color image retrieval system. Due to the characteristics of trademark images, while these approaches mentioned above may have achieved certain degrees of success in their particular applications, these approaches are not appropriate for identifying the markers in radiographs directly, because, compared to trademark images, radiographic images present much more complicated characteristics. Firstly, the radiographic images are grey level images, while trademark images are generally binary images or images with very limited color/grey levels. Secondly, the radiographs include more image contents, such as collimation areas, direct exposure area and diagnostic relevant regions. The marker is a very small region in the image and needs to be detected before recognition; otherwise its information can be totally buried by other image regions. However, for trademark images, since the trademark generally occupies the entire image, recognition can be directly performed on the image.
Accordingly, there exists a need for a method to automatically identify markers in radiographic images. Such a method should be robust and suited to accommodate variations in radiographs